Antigen Presentation Pathways: Key Mechanisms for T Cell Recognition and Immune Response Activation

Antigen recognition by T cells: Antigen presentation

~ APCs deliver an antigen-specific signal through the major histocompatibility complex (MHC)-peptide->T cell receptor (TCR) interaction on T cells.~ Note the cycle in the slide

Antigen presentation is a crucial step in the process of antigen recognition by T cells. This process involves the display of small pieces of protein (antigens) on the surface of antigen-presenting cells (APCs) to T cells. APCs can be dendritic cells, macrophages, or B cells.

Antigen presentation occurs in two different ways: the major histocompatibility complex class I (MHC-I) pathway and the MHC-II pathway. MHC-I pathway is used to present antigens derived from intracellular pathogens (such as viruses or tumor cells), while MHC-II pathway is used to present antigens derived from extracellular pathogens (such as bacteria or parasites).

In the MHC-I pathway, antigen-presenting cells engulf and digest the pathogen. The resulting peptides are then transported to the endoplasmic reticulum (ER) where they are loaded onto MHC-I molecules. The MHC-I molecules plus the antigen fragments are then transported to the cell surface where they are presented to CD8+ T cells. The T cell receptors (TCRs) on the surface of CD8+ T cells recognize the MHC-I-antigen complex and, if the TCR binds to the complex, it triggers the T cell to become activated and initiate an immune response to destroy the infected cells.

In the MHC-II pathway, antigen-presenting cells engulf extracellular pathogens and digest them in lysosomes. The resulting peptides are then loaded onto MHC-II molecules and transported to the cell surface for presentation to CD4+ T cells. The TCRs on the surface of CD4+ T cells recognize the MHC-II-antigen complex and, if the TCR binds to the complex, it signals the T cell to become activated and initiate an immune response to help eliminate the invading pathogen.

Overall, the interaction between antigens and MHC molecules is critical for the activation of T cells and the initiation of an immune response against foreign invaders.

More Answers: